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Thymus

The lymphoid organs, including thymus, spleen, lymph nodes and tonsils are part of the Lymphatic System. The lymphatic system serves a purpose in immune defense. Understanding this system is essential for the diagnosis of many pathological processes. This article deals with the structure, modes of action and a basic overview of the diseases of the lymphatic system.

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region of the paracortex being specialized
for T cells. Let’s now look at the thymus.
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Section here shows what the typical thymus
looks like. It has got a capsule andthymic lobules. The thymic lobule has within it a
dark staining cortex and a lighter stainingmedulla. And one feature of the thymus, one way
in which you can identify the thymus fromother organs is the fact that it contains
thymic or Hassall’s corpuscles which I’llshow you in high magnification. The thymus
is quite a complex organ, but quite easy todescribe and identify histologically, as I
just pointed out to you. On the left-handside is a diagram illustrating the compartments
of the thymus, the cortex and the medulla.
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What essentially happens is that the cortex
contains T cells undergoing education.
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And they undergo education in various units or
subdivisions of the cortex. And then theypass into the medulla where they undergo their
final education before then moving out tothe body. I sort of used the analogy that’s
like a school. The cortex is like grade schoolor primary school where you’re receiving
a certain education, and then you graduateand go to high school. Here in the cortex,
T cells undergo a certain amount of training.
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They go to the medulla then and then graduate
as fully mature immunocompetent T cells. But one differencehere is that only 2% ever graduate and move out
of the thymus medulla. The rest are apoptosedor digested by macrophages. They don’t complete
their education. They recognize maybe self-antigens etc.
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So the body gets rid of them. On
the diagram on the left, you can see rathera complex array of cells. There are six major
cells that live in the thymus cortex and thethymus medulla. And they’re called epithelioreticular
cells. They’re derived from the epithelium,and as I said before, they don’t secrete
reticular fibres, they’re not reticularcells. The type I are the ones that are on
the surface of the cortex. These are the onesthat actually separate the trained or the
educating thymus T cells from connective tissue.
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These cells form a barrier around all the
connective tissue, the capsule, the trabeculae,and even the connective tissue around the
blood. So they had jobs to separate thesethymus cells or T cells undergoing education.
Type II cells are in the network of the cortex.
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They compartmentalize the cortex into areas
such as you can see shown on the diagram.
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And I’m not going to go into all the details
of the physiology of the thymus here,I’m just pointing out some of the histological
or structural features. The type II cellsare also involved in training, educating the
T cells. Type III cells are also involvedin education of the T cells, and they form
a barrier between the cortex and the medulla,a very strong barrier to stop cells from passing
from the cortex into the medulla that haven’tgone through the proper training program.
The type IV cells are in the medullary side.
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And what they do is they form a seal again
on the medullary side so you have two cells,the type III and the type IV, making this
barrier from the cortex into the medulla.
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Notice that the medullaries in most of that
section you see on the right-hand side, lighterstaining, lighter staining because there're
far less cells in there because as I said,only 2% of cells ever make it out from the
thymus medulla. The rest die. And then finally,get the type VI. These are called the Hassall’s
corpuscles. They’re the ones that are probablyvery aged epithelioreticular cells, and they
form these spiral coils very easy to identifyin the medulla, and therefore, they're characteristic
of the thymus, and as I said earlier, enablesyou to identify thymus compared to other organs.
And finally, we need to say something about theblood-thymus barrier. We don’t want antigens
and other components of the blood, self-antigens,interfering with the education of these T cells.
So, these type II and type I epithelioreticularcells form a barrier, a wrap around these
capillaries. And along with the endothelialcells and the basal lamina and macrophages,
prevent the contamination of the educatedT cells or T cells undergoing education from
being exposed to those antigens.
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Macrophages live within the thymus, the cortex,
and the medulla. And they are busy wrapping up orphagocytosing cells that don’t pass the grade.
So again, in a review, make sure you’reaware of the structure of the cortex and the
thymus medulla, and the role or function ofthese special epithelioreticular cells. Finally,
let’s look at the spleen. A lymph node filters

About the Lecture

The lecture Thymus by Geoffrey Meyer, PhD is from the course Lymphoid Histology.

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